In 2007, the cesarean delivery rate was 31.8 percent—the highest level ever reported for the United States (Hamilton and co-workers, 2009). According to the American College of Obstetricians and Gynecologists (2003), approximately 60 percent of primary cesarean deliveries in the United States are attributable to the diagnosis of dystocia. Roy (2003) has proposed that this high frequency results from environmental changes that are developing more rapidly than Darwinian natural selection. Humans are poorly adapted to the affluence of the modern diet, and one result is dystocia. Evidence in support of this comes from Barau and associates (2006), who analyzed prepregnancy body mass index (BMI) and the risk of cesarean delivery. They studied 16,592 singleton births and reported a linear association between BMI and cesarean delivery. This has been similarly shown by others (Leung, 2008; Nuthalapaty, 2004; Roman, 2008; Treacy, 2006; Wilkes, 2003, and all their colleagues). As further discussed in Chapter 43, Getahun and co-workers (2007) reported that obesity is associated with an increased cesarean delivery rate. Interestingly, these researchers found that a decrease in weight from obese to normal eliminates this risk.

Abnormalities of the expulsive forces. Uterine contractions may be insufficiently strong or inappropriately coordinated to efface and dilate the cervix—uterine dysfunction. Also, there may be inadequate voluntary maternal muscle effort during second-stage labor.

More simply, these abnormalities can be mechanistically simplified into three categories that include abnormalities of: the powers—uterine contractility and maternal expulsive effort; the passenger—the fetus; and the passage—the pelvis. Common clinical findings in women with these labor abnormalities are summarized in Table 20-1.

Dystocia Definitions

Combinations of the abnormalities shown in Table 20-1 often interact to produce dysfunctional labor. Today, expressions such as cephalopelvic disproportion and failure to progress often are used to describe ineffective labors:

The expression cephalopelvic disproportion came into use prior to the 20th century to describe obstructed labor resulting from disparity between the size of the fetal head and maternal pelvis. But the term originated ...